1. Field of Invention
The present invention relates generally to methods and apparatus for milling windows in well casings in the downhole environment whenever the trajectory of a well should be modified after a casing or liner has been set in a well or when one or a plurality of branches are built from a parent well. More particularly, the present invention concerns a method and apparatus for milling casing windows which ensures predictable milling so that the resulting casing window will be of predetermined dimension, contour geometry, location and orientation. Even more specifically, the present invention provides for stabilized rotation and efficiently controlled guiding of a pilot mill having articulated and rotary driven relation with a substantially rigid string mill, especially during initiation of casing milling, to ensure efficient deflector controlled guiding of the pilot mill and guiding of the string mills by the pilot mill, to ensure precisely controlled formation of a casing window by the pilot mill and string mills. The present invention also concerns a casing window milling system incorporating an articulated pilot mill having the capability for controlling its amplitude of relative misalignment with a substantially rigid milling shaft and having rotary driven relation with the milling shaft during initiation of casing milling and during initial pilot boring into the subsurface formation from the casing window.
2. Related Art
Casing windows are required whenever the trajectory of a well should be modified after a casing or a liner has been set in a well or when one or a plurality of branches are built from a parent well.
A casing window is generally performed with a combination of mills mounted on a mandrel at the bottom end of a drill string and wedging between the casing and a deflection tool called the whipstock. The whipstock is generally set in the hole in combination with the first milling run. The window may be completed in one single operation in the hole or in multiple runs. The peripheral surface of mills is generally covered with abrasive or cutting inserts made of hard material such as sintered tungsten carbide compounds brased on a steel mandrel. The hardness of the whipstock is generally designed so minimum wear will be generated by the rotation of mills peripheral surface onto the whipstock face while the assembly is pushed and rotated against the casing wall under deflecting action of the whipstock. However the milling action generally results from unbalanced pressures between respectively the mill(s) and the whipstock on one hand and the mill(s) and the casing wall on the other hand.
In high inclination condition, the whipstock face is generally oriented upward and therefore forces applied by the mill(s) onto the whipstock face increase with the increasing weight component of the milling string. Although a whipstock is expected to support some milling damage, how much whipstock material is left after milling has been preformed is difficult to predict. In such case the success of whipstock retrieval may become risky and lead to lost time and additional contingency and sometimes to the loss of the bottom section of the well.
The lack of control on the window geometry is another major disadvantage of conventional window milling techniques and makes some lateral branching techniques inapplicable or more complex. Most windows show a lower section directed sideways with respect to the hole axis. How much this xe2x80x9cwalk awayxe2x80x9d affects a window is hardly predicable and depends on several factors like well inclination, pilot mill size and shape, mill cutting structure, weight on bottom hole assembly, whipstock hardness and orientation.
When the formation surrounding the well casing being penetrated by the window bore is well consolidated, it is desirable that the pilot mill have a geometry enabling it to be efficiently guided along an intended trajectory by the wall surface of the wellbore being formed. When the formation surrounding the wellbore is not well consolidated, a pilot mill which has a freely articulated and rotary driven connection with a substantially rigid milling shaft could be subject to forces that might tend to change its course from the intended trajectory. If the pilot mill should be suddenly articulated when encountering some unusual structure in the downhole environment, the pilot mill or its articulated connection with the milling shaft could become damaged, perhaps to the extent of being separated from the milling shaft. It is desirable therefore to provide a casing window milling system having an articulated pilot mill and also having a mechanism for controlling the amplitude of relative misalignment of the pilot mill relative to the axis of rotation of the milling shaft. This pilot mill amplitude control feature will permit the pilot mill to be efficiently deflected so as to follow the slope of the deflecting tool without damaging the deflecting tool and will permit the pilot mill to be constrained in a coaxial relationship with the milling shaft so as to be guided by the milling shaft after the pilot mill has passed a point on the deflecting tool where self guiding of the pilot mill can no longer be ensured. Thus it is desirable to provide a casing window milling tool which incorporates a locking or restraining mechanism which can be actuated mechanically or hydraulically to lock the pilot mill in co-axial, stabilized relation with the milling shaft.
It is a primary feature of the present invention to provide a novel method and apparatus for predictable milling of casing windows which employs a rotary milling tool having an articulated pilot mill provided with cutting means only on its forward axial end so that the pilot mill is capable of cutting only on the forward axial end thereof and will not cut or substantially erode away a deflection element that is utilized to guide the pilot cutter;
It is another feature of the present invention to provide a novel method and apparatus for predictable milling of casing windows which utilizes an articulated pilot mill not only for pilot hole cutting but also for efficiently guiding other milling cutters of the apparatus during milling activities so that the geometry and location of the resulting casing window will conform specifically to plan and will not be varied by other factors during milling;
It is also a feature of the present invention to provide a novel method and apparatus for predictable milling of casing windows which employs guide means such as a tubular guide bearing to render the pilot mill extremely stable during initial forming of the casing window;
It is another feature of the present invention to provide a novel method and apparatus for predictable milling of casing windows which utilizes an articulated pilot mill having a non-milling periphery for guided engagement with an inclined guide surface of a deflecting device and having a forward milling end for milling a pilot window bore through the well casing and into the surrounding formation;
It is also a feature of the present invention to provide a novel method and apparatus for predictable milling of casing windows wherein a pilot mill is employed which has articulated driven connection with a substantially rigid string mill and which is adapted for non-milling engagement with an inclined guide surface and is adapted for pilot window milling engagement with the casing of a well;
It is a feature of the present invention to provide a well casing milling system incorporating a pilot mill having articulated driven connection with a substantially rigid string mill shaft wherein the articulated driven connection comprises a universal joint which transmits torque and axial load from the substantially rigid string mill shaft to the pilot mill;
It is also a feature of the present invention to provide a novel casing window milling system having a pilot mill that has articulated rotary driven connection with a substantially rigid milling shaft by means of a universal joint and wherein the universal joint incorporates an articulation control mechanism for adjusting the amplitude of angular misalignment of the pilot mill relative to the milling shaft between a maximum allowable angle and a coaxial relationship and for locking the pilot mill at the selected amplitude of angular misalignment;
It is another feature of the present invention to provide a well casing milling system incorporating a pilot mill and a substantially rigid string mill shaft and means for decoupling the bending moment that would otherwise be transmitted between the pilot mill and string mill shaft as the pilot mill is diverted from the longitudinal axis of the well casing to the inclined path of the guide surface of the deflector tool;
It is an even further feature of the present invention to provide a well casing milling system incorporating a deflecting tool having an upper guide bearing to provide an articulated rotary driven pilot mill of a milling assembly with precise guiding during initial casing window milling to ensure rotary stabilization of the pilot mill and ensure proper orientation and direction of the pilot bore;
It is a feature of the present invention to provide a well casing milling system incorporating a pilot mill having articulated driven connection with a substantially rigid string mill shaft and wherein the articulated rotary driving connection defines a flow passage through which a suitable fluid may be pumped for cooling or otherwise enhancing the casing window milling operation;
It is a feature of the present invention to provide a well casing milling system incorporating a pilot mill having articulated driven connection with a substantially rigid string mill shaft and wherein the pilot mill defines a non-milling substantially cylindrical guiding periphery and the articulated rotary driving connection defines the axis of rotation of the pilot mill and is located within and intermediate the axial length of the pilot mill to provide for stability and guidance thereof;
It is another feature of the present invention to provide a well casing milling system incorporating a deflecting tool which is set within the well casing and which defines an inclined guide surface for non-milling engagement by an articulated pilot mill of a casing window milling assembly and which deflecting tool defines a passage through which fluid may be caused to circulate and well tools may be passed for conducting other well activities with the deflecting tool in place or for retrieval of the deflecting tool from the well casing;
It is a feature of the present invention to provide a well casing milling system incorporating a pilot mill having articulated driven connection with a substantially rigid string mill shaft and employing a rotary drive means having articulated driving connection with the substantially rigid string mill shaft, which rotary drive means may take the form of a positive displacement motor, turbine or other equivalent power source and which rotary drive means may be rotated by a drill string for enhancing the power and/or speed of the milling system;
It is another feature of the present invention to provide a novel method and apparatus for predictable milling of casing windows and has a pilot mill which has articulated driven connection with a substantially rigid milling shaft having string mills and which provides radial force to the rigid shaft and string mills causing the string mills to penetrate into the casing without substantial wear of the guide face of the deflection tool;
It is also a feature of the present invention to provide a novel method and apparatus for predictable milling of casing windows which incorporates a deflecting tool which is set within the well casing and a milling assembly having a substantially rigid milling shaft and a pilot mill having articulated rotary driven connection with the milling shaft and wherein the milling assembly and the deflection tool may be releasably interconnected during running operations to ensure single pass installation and desired initial relative positioning of both the deflecting tool and milling assembly before the casing window milling operation is initiated;
It is an even further feature of the present invention to provide a novel method and apparatus for predictable milling of casing windows which employs an elongate milling tool having sufficient stiffness to prevent or minimize its deflection during milling so that the resulting casing window will have precisely and predictably determined characteristics of window dimension, window contour geometry and location;
It is also a feature of the present invention to provide a novel method and apparatus for predictable milling of casing windows which employs deflection tool establishing a substantially tubular pilot mill guide or pilot mill and rotary drive motor guide for guiding the articulated pilot of the window milling tool and wherein a portion of the tubular pilot guide is partially milled by succeeding window mills to form the deflecting tool with a predictable guide surface geometry that is suitable for guiding well tools from the main well bore through a casing window and into a lateral bore; and
It is an even further feature of the present invention to provide a novel method and apparatus for predictable milling of casing windows which incorporates a deflecting tool and milling tool which enable guided movement of the milling tool and its rotary drive motor and rotary stabilizer within a guide passage of the deflecting tool; and
It is also a feature of the present invention to provide a novel method and apparatus for predictable milling of casing windows which is design to enable a deflecting tool and a casing window milling tool to be run into a well casing as a unitary assembly and after milling of a casing window, to be extracted from the well casing as an assembly.
Briefly, a downhole casing window milling assembly embodying the principles of the present invention is composed of a rotary positive displacement motor, a hollow rotary driving articulation connected to the motor bit box on its upper end and to a substantially rigid milling shaft on its lower end, a pilot mill having articulated connection with the substantially rigid milling shaft, a deflection tool releasably connected to the bottom of the milling tool and an anchoring device at the very bottom which additionally provides for location and orientation of the casing window milling system within the well casing.
The rotary positive displacement motor drives the milling assembly through an articulated joint such as a universal joint or a short flex joint which also defines a flow passage. The purpose of such articulation or short flex joint is to decouple, cancel or minimize bending moments that could be transmitted by the milling assembly to the motor bearings while still allowing fluid to circulate to the bottom of the milling assembly. If desired, the rotary drive motor can eventually include two power sections to provide additional torque without creating additional conveyance constraints in high dog leg severity wells.
The downhole motor can be also a turbine or other alternative downhole rotary power generation wherever the mechanical power source will be most appropriate without noticeably affecting the basic benefit of the milling equipment. The downhole motor and its rotational stabilizer can also be adapted for passing through the deflecting tool and to be guided by the deflecting tool when the deflecting tool incorporates a tubular guide.
Although use of downhole rotating power source such as positive displacement motors provide better milling performance in deviated or horizontal wells, the bottom milling tool may be alternatively powered by or in combination with a conventional rotary drill string. While using a downhole power source, the drill string may be rotated to provide additional mechanical power to the milling tool and also to minimize the effect of dragging forces and thus provide better control of milling tool penetration.
The casing window milling assembly is composed of a plurality of string mills mounted on a substantially rigid hollow milling shaft. A pilot mill is mounted for articulation at the bottom end of the milling shaft and is rotated and moved axially by the milling shaft. The pilot mill is of generally cylindrical configuration and defines a generally cylindrical outer peripheral surface which establishes a non-milling, guided relationship with the inclined guide surface of the deflecting tool. The pilot mill has a milling face only at its forward end and has no abrasive material on its outer periphery so that the deflecting tool is not subject to significant milling action by the pilot mill as the pilot mill is rotated and guided during window milling. The pilot mill is articulated within a small angular amplitude relative to the milling shaft so it can spin along an axis parallel to the inclined guide face of the deflection tool and be guided without milling the guide face of the deflection tool, unlike conventional casing window milling tools which typically having milling contact with the deflection tool and thus tend to remove at least a portion of the guide face during milling. The milling shaft is provided with at least one and preferably two or more string mills, such as a gauging mill and a reaming mill, for example, which are each typically of greater diameter than the diameter of the pilot mill. The initial string mill is mounted to the milling shaft at a relatively short distance from the pilot mill so most of the opening milled in the well casing will be made with the initial string mill. Optionally, one or several reaming mills can also be mounted on the milling shaft above the first string mill. In most common situations, casing windows are of full size, meaning that the diameter of a cylinder passing through the window is substantially equal to the casing inside diameter. In this case the outside diameter of the pilot mill is smaller than that of the string mill(s) which typically have a diameter that is very close to the drift diameter of the casing. The milling system can incorporate a locking or restraining mechanism for controlling the amplitude of misalignment of the pilot mill relative to the milling shaft from a coaxial relationship to a relationship permitting a maximum degree of allowable articulation. This feature permits the pilot mill to be efficiently guided along the slope of the deflecting tool or whipstock during initial casing window milling and permits guiding of the pilot mill to be controlled by the milling shaft when the pilot mill has moved along the guiding face of the whipstock to a point that its efficient self guiding can no longer be ensured. In one suitable form the locking or restraining system may take the form of a hydraulic piston actuated mechanism which is maintained in a release position by captured hydraulic fluid within a closed chamber. The hydraulic fluid may be released in any suitable manner, such as by breaking of a frangible element or by pressure responsive opening of a release valve to permit spring urged movement of the hydraulic piston to a position causing restraint or locking of the articulated connection between the pilot mill and the milling shaft. When so restrained, the pilot mill will be guided along the intended trajectory by its coaxial or axial misalignment controlled relation with the milling shaft and with its trajectory being controlled by the milling shaft. Moreover, under conditions where unusual forces are encountered that might tend to deflect the pilot mill from its intended course the locking or restraining mechanism will ensure that the pilot mill will maintain its intended trajectory.
In the case of undersize windows, meaning that the diameter of a cylinder passing through the window is substantially smaller than the casing inside diameter, the diameter of the pilot mill may be equal to the diameter of the string mills. This is generally the case of window milling in a production liner/casing which requires the milling tool to be passed through a production tubing.